This invention concerns projection television systems, and is particularly directed to system in which a rear projection screen, storable in the system cabinet, is elevatable for viewing.
A jack-in-the-box projection television receiver comprises cabinet means for enclosing the components of the receiver, and includes lid means hinged at the back of the cabinet. The receiver comprises vertically arranged, elevatable rear projection screen means selectively storable in the cabinet. Image source means provides for forming a television image. Stationary projection means provide for projecting an aerial image of a television image formed by the image source means along a folded optical path onto the screen means when the optical path is erected and the screen means is located a predetermined image projection distance from the source means. The receiver further includes means for elevating the screen from a first receiver-inoperable position wherein the screen means is completely enclosed within the cabinet and the receiver is ultra-compact, to an elevated receiver-operable position wherein the screen means is emerged from the cabinet. As a result, the optical path is extended to a length equal to the predetermined image projection distance whereby the projection image is coincident with the screen.
The overall size and configuration of a projection television system is dictated primarily by its optical system. The optical system comprises the means for forming a television image and projecting an aerial image of the television image along an optical path onto a viewing screen. The screen may be of the "front-projection " type wherein the image is cast on the screen and reflected back to the viewer. Or the screen may be of the "rear projection" type wherein the image is cast upon a rear surface of a translucent material, whereupon the image appears on the front of the screen.
The cabinet design constraints dictated by prior art optical systems have resulted in projection television systems which are massive in size and deep from front to back relative to a conventional console television receiver which utilizes a 25-inch cathode ray picture tube. In addition, projection television systems in general are cumbersome and may require setting up preliminary to operation, a factor that is not appealing to a homeowner accustomed to pressing a single button to activate his familiar console television set.
An example is a two-piece projection television system wherein the projection unit is physically separated from a remotely located viewing screen. FIG. 1 is a simplified schematic of such a system. A television image is formed on the cathodoluminescent screen of a cathode ray tube 1, for example. Projection lens 2, contained in a free-standing cabinet 3 along with cathode ray tube 1, provides for projecting an aerial image of the television image along optical path 4 onto the surface of curvilinear front projection viewing screen 5. The distance between cabinet 3 and screen 5 may exceed eight feet. Preparing the system for operation requires the careful location of cabinet 3, which is usually caster-mounted, in relation to the screen 5. The brightness of screen 5, made curvilinear to focus the image for maximum brightness, diminishes quickly off the screen axis, making it necessary for viewers to cluster closely around the axis in order to see an acceptable picture.
Another basic configuration is a system, shown schematically in FIG. 2, wherein a forwardly tilted, curvilinear, front-projection viewing screen 6, which may be demountable, is mounted uprightly at the back of a cabinet 7. An optical assembly 8, which commonly includes three cathode ray tubes and associated focusing lenses as depicted, is housed in the cabinet 7. The optical assembly 8 casts a plurality of television images on a mirror 9 which, in prepartation for operation, is manually transported on slide means forwardly away from the screen 6 and toward the viewer to a fixed position. The purpose of the mirror 9 is to reflect the images onto the screen where they are brought into coincidence. The necessary projection distance is thus provided. The optical path is noted as being "folded" to provide an optical system that occupies a smaller area, and hence makes possible a smaller cabinet than would otherwise be necessary. The larger the viewing screen, however, the longer the projection optical path and the closer to the viewer the mirror must extend. In one product of this type, for example, wherein the viewing screen is sixty inches in diagonal measure, the mirror is slide-mounted for extension toward the viewer, and the mirror must be transported nearly two feet toward the viewer from the cabinet proper. As a result, the receiver is very deep from front to back during operation. An example of a system of this type is disclosed in U.S. Pat. No. 4,239,894 to Tokumaru et al.
A third representative type of projection television system is a rear-projection system in which the image is projected on the rear of the viewing screen for viewing by an observer located at the front of the screen. An example is the Model PR4800SW projection television receiver manufactured by Quasar Electronics Corp., Franklin Park, Illinois. The system is contained in a single large cabinet the height of a china cabinet. The screen is exposed for viewing by opening the doors of the upper section of the cabinet to reveal a screen of 45-inch diagonal measure. Another example of a television set with a fixed rear projection screen is disclosed in U.S. Pat. No. 4,177,484 to Boje.
In U.S. Pat. No. 3,115,544 to Marley, there are disclosed several configurations of optical system intended to provide a color television receiver of compact dimensions. A receiver having multiple light reflectors common to the converging light paths and position to provide multiple folds in such light paths is shown and described. The receiver further includes beam deflecting means for each of three cathode ray tubes developing a scanning raster whose projected image is pre-distorted by electronic means to provide an undistorted viewing image when the image of off-axis tubes are converged. Magnetic deflection yoke means are relied upon to provide distortion-correction.